1. Field of the Invention
The invention relates to a process and an apparatus for producing a fibrous web, in particular a tissue web.
2. Description of the Related Art
Tissue paper ideally has high absorbency or a high water absorption capacity in conjunction with a high tear resistance. The absorbency and the water absorption capacity are determined substantially by the volume and porosity of the tissue paper.
In order to increase the volume, it has already been proposed to press the tissue paper web only zonally during its production, in order, in addition to the pressed regions of higher tear resistance, and to obtain more lightly pressed or unpressed more voluminous regions.
During the production of tissue paper, in a last drying step, the tissue web is led over the circumferential surface of a heated Yankee drying cylinder, before the finished product is creped off the latter. While the tissue web is being led over the Yankee drying cylinder, it is held by a fabric.
In particular during the production of tissue paper with voluminous regions which have been compressed only slightly during the dewatering, there is, however, the problem that the tissue paper comes into contact with the hot circumferential surface of the Yankee drying cylinder with an excessively low dryness. This problem occurs to a greater extent at high machine running speeds, since here the dewatering times are reduced further and the voluminous regions accordingly carry still more moisture with them.
On account of the excessively low dryness, during contact of the tissue web with the heated circumferential surface of the Yankee drying cylinder, water vapor is produced between the hot circumferential surface and the tissue web, which can lead to the web lifting off the roll circumferential surface.
As a result of the tissue web lifting off the roll circumferential surface, it is possible for problems to occur with account to the runnability, up to breaking of the tissue web.
Furthermore, on account of the formation of water vapor between the tissue web and the heated circumferential surface of the Yankee drying cylinder, it is possible for the formation of bubbles and holes in the tissue web to occur.
It is already known to coat the drying or Yankee cylinder in order to counter the problems occurring during a transfer of the fibrous web from a TAD (through air drying) fabric to the surface of the Yankees cylinder. In addition, a doctor arrangement has already been proposed in which a doctor crepes the fibrous web and doctors it off the Yankee cylinder, and at least one further doctor is provided for the purpose of removing a layer of the roll coating containing dirt.
At present, there exist two different processes for producing tissue paper. Firstly, there is the conventional tissue production process, in which the fibrous web is formed, pressed and dried on the Yankee cylinder. Secondly, there is what is known as the TAD production process (TAD=through air drying), in which the fibrous web is dried between the sheet forming zone and the press section by way of an air stream. This method is associated with a high paper quality.
To address the aforementioned problems, various parameters, in particular those which relate to the region of the Yankee cylinder, must be chosen in a suitable way.
Typical values for some known parameters are listed in the following table:
TABLE 1Quantity of roll coatingBlade ormaterialdoctor loadingmg/m2; mL/minkN/mConventional tissue machine1-3; 15-25~2.52-3TAD machines5-15; 80-100~7.06-7
The high quantity of coating material previously required to coat the Yankee cylinder is obviously associated with economic disadvantages. The same applies to the relatively high blade or doctor loading previously required.
What is needed in the art is an improved process and an improved apparatus of the type mentioned at the beginning. What is needed in the art is an improved process and an improved apparatus which ensures the highest possible quality of the tissue paper and, at the same time, ensures that this high product quality can also be achieved with a lower quantity of coating material for the Yankee cylinder and a lower blade or doctor loading.